The interactions between plants and various soil life forms are very complex, in some instances helpful to the plant and in other instances deleterious to the plant. Fungi harmful to plants (fungal pathogens) include fungal species from a wide variety of genera, including Fusarium, Pythium, Phytophthora, Verticillium, Rhizoctonia, Macrophomina, Thielaviopsis, Sclerotinia and numerous others. Plant diseases caused by fungi include pre- and post-emergence seedling damping-off, hypocotyl rots, root rots, crown rots, vascular wilts and a variety of other symptoms. Nematodes harmful to plants (nematode pathogens) include nematode species from the genera Meloidogyne, Heterodera, Ditylenchus, and Pratylenchus. Plant diseases caused by nematodes include root galls, root rot, lesions, "stubby" root, stunting, and various other rots and wilts associated with increased infection by pathogenic fungi. Some nematodes (e.g., Trichodorus, Lonoidorus, Xiphenema) can serve as vectors for virus diseases in a number of plants including Prunus, grape, tobacco and tomato.
Plant disease is the exception rather than the rule, as many plant pathogens express a virulent phenotype only on one or a limited number of host species. Pathogens inoculated onto a non-host species either lack the ability to grow and infect that plant, or following ingress invariably encounter a successful resistance mechanism. Interactions with host species exhibit a range of specificities dependent on the mechanism of the pathogen. One type of pathogen is the unspecialized "thug", which is necrotrophic and damages the host through production of toxins and/or enzymes. This pathogen is often equipped with inactivation or avoidance mechanisms to deal with host defenses. In contrast, "con men" pathogens grow biotrophically, avoiding serious host damage, at least in the early stages. Unlike thugs, con men pathogens do not activate host defenses nonspecifically.
For specialized pathogens having no avoidance mechanisms, host defenses are usually effective if induced. In specific interactions, following attempted infection by the pathogen, molecular signals determine whether the interaction will be incompatible or compatible. In an incompatible interaction (host resistant, pathogen avirulent), early molecular recognition is followed by rapid expression of defense responses. In a compatible interaction (host susceptible, pathogen virulent), the pathogen eludes the plant's surveillance mechanisms and disease generally ensues.
Various approaches have been utilized for attempting to control deleterious fungi and nematodes. One method is application of certain naturally occurring bacteria which inhibit or interfere with fungi or nematodes. See, for example, K. F. Baker and R. J. Cook, Biological Control of Plant Pathogens, Freeman and Co. (1974), for a description of fungi and nematodes and their interaction with plants, as well as a description of means for biological control of fungal and nematode pathogens. Another method is breeding for resistance, which is primarily focussed on the manipulation of minor resistance genes which make small quantitative contributions to the overall resistance of the plant.
Lytic enzymes have been individually transduced to form single-gene transgenic plants. For example, transfer of a tobacco basic vacuolar chitinase gene under the control of the CAMV35S promoter into the closely related species, N. sylvestris, did not give effective protection against C. nicotianae, even in transgenic plants exhibiting constitutively high levels of chitinase activity (Neuhaus et al., Plant Mol. Biol., 16:141-151, 1991). See also U.S. Pat. No. 4,940,840 to Suslow et al.
Recombinant bean chitinase has been expressed in transgenic tobacco seedlings (Broglie et al., Science, 254:1194-1197, 1991). The seedlings of the chitinase-containing transgenic plants have an enhanced, but not complete, resistance to a single species of fungi. Thus, it is desirable to obtain plants that have higher resistance levels to fungal pathogens than existing plants.